The present invention is related to data transmission and particularly transmission of data over cable TV systems.
The initially installed cable systems for the transmission of television signals were primarily one-way transmission systems wherein the transmission of the TV signal was made through repeaters, periodically installed in the cable system, which amplified the signal. Transmission was one-way from the "head-end" to the subscribers. Two-way systems have now been specified (required) and all systems installed after 1977 are "two-way capable", although not "two-way ready".
Two-way systems employ diplexers in the repeaters. The diplexer breaks up the service spectrum into two components, a low component of five magahertz to thirty megahertz for return signaling, and a high component of fifty-four megahertz to four hundred megahertz, for transmission to the subscriber. With current two-way ready systems both the high band and the low band are amplified. Two-way transmission of data takes place by "polling" with an outgoing signal, multiplexed in with the TV signals, transmitted to the subscriber on the high band and the return signal transmitted on the power amplified low band. With current two-way "polling" systems, responses or return signals from the various subscribers are transmitted on a single time shared return channel.
A clear disadvantage of a polling system is that if one of the remote units in the transmitters locks up and it stays on, the entire system is jammed. No one else can transmit. A more serious disadvantage arises from the use of amplification. If power is lost at any repeater in the system, the signal is lost and cannot be received or decoded by the "head-end" modem. This is an obvious problem with informational type data and particularly a problem when such a system is employed as an alarm system to indicate fire, unauthorized entry or the like. It should also be obvious that, with amplification, it is only possible to transmit in one direction on the low band.